1. Field of the Invention
The present invention relates to a magnetic recording medium mounted on various magnetic recording apparatuses.
2. Description of the Related Art
A perpendicular magnetic recording medium has been employed as a technique for achieving higher magnetic recording density. A perpendicular magnetic recording medium includes at least a non-magnetic substrate and a magnetic recording layer formed from a hard magnetic material. The perpendicular magnetic recording medium may optionally further include a soft magnetic underlayer that performs a role of concentrating a magnetic flux generated by a magnetic head on the magnetic recording layer, an underlayer for aligning the hard magnetic material of the magnetic recording layer in a desired direction, a protective layer that protects the surface of the magnetic recording layer, and other layers.
As the hard magnetic material, a granular magnetic material in which a non-magnetic material, such as SiO2 or TiO2, is added to a magnetic alloy material, such as CoCrPt or CoCrTa, has been proposed. For example, a film of CoCrPt—SiO2 granular magnetic material has a structure in which SiO2, which is a non-magnetic material, is segregated so as to surround CoCrPt magnetic crystal grains. Here, the individual CoCrPt magnetic crystal grains are magnetically decoupled by SiO2 which is the non-magnetic material.
In recent years, there is a need to further improve recording density of a perpendicular magnetic recording medium. Specifically, a perpendicular magnetic recording medium capable of achieving linear recording density of 1500 kFCI (field changes per inch) or higher is required. As means for realizing such a linear recording density, reducing the size of magnetic crystal grains in the granular magnetic material has been studied. However, a reduction in the size of the magnetic crystal grains results in deterioration in thermal stability of recorded magnetizations (recorded signals). In order to compensate for the deterioration in thermal stability, it is required to improve crystalline magnetic anisotropy of the magnetic alloy material in the granular magnetic material.
A L10-based ordered alloy is one of the materials having the required high crystalline magnetic anisotropy. On the other hand, the non-magnetic substrate of the magnetic recording medium is formed using aluminum or glass in order to satisfy the required substrate characteristics such as strength or impact resistance. An underlayer is important when forming a L10-based ordered alloy film is formed on the surface of the non-magnetic substrate. This is because the crystals of the L10-based ordered alloy need to have (001) orientation (the [001] axis of the crystal needs to be perpendicular to the principal surface of the non-magnetic substrate) in order to realize high crystalline magnetic anisotropy.
In general, in order to realize desired crystal orientation of a L10-based ordered alloy, MgO or SrTiO3 having an appropriate lattice misfit with respect to the L10-based ordered alloy has been used as the underlayer. For example, Japanese Patent Application Publication No. 2001-101645 (Patent Document 1) indicates that in a structure in which a soft magnetic material layer, a non-magnetic material layer, and an information recording layer formed from a L10-based ordered alloy are sequentially formed, when MgO is used as the non-magnetic material, the crystallinity, the crystal orientation, and the magnetic characteristics of the information recording layer formed from the L10-based ordered alloy are improved (see Patent Document 1). Alternatively, WO-2004/075178 (Patent Document 2) discloses a magnetic recording medium having a stacked structure including a soft magnetic underlayer, a first orientation control layer formed from a magnetic material, a second non-magnetic orientation control layer, and a magnetic recording layer containing crystal grains having a L10 structure (see Patent Document 2). However, the thin films, i.e., the information recording layer and the magnetic recording layer, of the L10-based ordered alloy disclosed in these documents do not have a granular structure. Thus, the recording density (resolution) of the magnetic recording signal is approximately 220 kFRPI (flux reversals per inch, see Patent Document 1) and 400 kFCI, see (Patent Document 2).
In order to further improve the recording density (resolution), a thin film of L10-based ordered alloys having a granular structure capable of reducing the size of magnetic crystal grains and improving magnetical decoupling between the magnetic crystal grains while securing the crystallinity and crystal orientation of the thin film of L10-based ordered alloys has been studied. For example, Japanese Patent Application Publication No. 2004-152471 (Patent Document 3) discloses a FePt—C thin film of L10-based ordered alloys having a granular structure including FePt magnetic crystal grains and C-based non-magnetic grain boundaries, formed on a magnesium oxide (MgO) substrate using a sputtering method. Moreover, Japanese Patent Application Publication No. 2008-091009 (Patent Document 4) indicates that a thin film having a granular structure containing magnetic crystal grains of a L10-based ordered alloy (FePt or the like) is obtained according to a sputtering method which uses a substrate heated to 650° C. or higher. Further, Published Japanese Translation of PCT Application No. 2010-503139 (Patent Document 5) discloses a magnetic recording medium having a structure including a substrate, an underlayer, a buffer layer, and a magnetic recording layer, in which the underlayer has a lattice misfit of 3% to 10% with respect to the magnetic recording layer and the buffer layer has (002) orientation whereby the magnetic recording layer having a granular structure including magnetic crystal grains formed from a L10-based ordered alloy and non-magnetic grain boundaries formed from additives can be formed at a temperature lower than 400° C.
In the disclosed method of forming the thin film of L10-based ordered alloys, which utilizes the benefits of the lattice misfit of the underlayer and the orientation of the buffer layer, when a FePt material containing 15 vol % of C is deposited on a substrate heated to 350° C., a granular structure having FePt grains having an average size of 7 nm and C-based non-magnetic grain boundaries having a width (the gap between adjacent FePt grains) of 1 nm is obtained (see Patent Document 5). However, the present inventor has found that when a Cr-based underlayer having a thickness in the range of 5 nm to 60 nm and a buffer layer formed from MgO or Pt having a thickness in the range of 2 nm to 8 nm were used, the width of FePt magnetic crystal grains and C-based non-magnetic grain boundaries in the granular structure changed depending on the amount of C added to the FePt material, the substrate temperature, and the thickness of the thin alloy film to be formed. In order to improve magnetical decoupling of FePt magnetic crystal grains in the granular structure, it is effective to increase the substrate temperature and to increase the amount of C added. However, it was understood that an increase in the substrate temperature promoted coupling of adjacent FePt magnetic crystal grains to increase the grain size. Moreover, it was understood that an increase in the amount of C added resulted in a decrease in magnetization strength (Ms) (that is, a decrease in the strength of magnetic signals during reading) of the thin alloy film (the magnetic recording layer). On the other hand, in order to improve the magnetization strength (Ms) of the thin alloy film (the magnetic recording layer), it is effective to increase the thickness of the thin alloy film (the magnetic recording layer). However, it was understood that an increase in the thickness of the thin alloy film (the magnetic recording layer) resulted in an increase in the size of the FePt magnetic crystal grains in the granular structure. From the above, in the disclosed configuration, it was difficult to simultaneously realize a further decrease in the size of magnetic crystal grains, more satisfactory magnetical decoupling between the magnetic crystal grains, and higher magnetization strength (Ms) characteristics to thereby obtain satisfactory signal characteristics required for high-density magnetic recording.
The present invention has been made in view of the above-described problems and an object thereof is to provide a magnetic recording medium including a magnetic recording layer having a granular structure, capable of achieving satisfactory signal characteristics at high magnetic recording density.